Current methods for ligament reconstruction often involve tissue transfer procedures, wherein a tissue is transplanted from one part of the body to the site of the damaged ligament in order to reconstruct the damaged ligament. An example of such a procedure in the knee is the reconstruction of the anterior cruciate ligament by using a portion of the patellar tendon. Other tendons such as the semitendinosus tendon, and connective tissues such as fascia lata, are sometimes used to reconstruct the damaged ligament.
A problem associated with these methods for ligament reconstruction relates to the loss of strength of the transferred tissue in the early post-operative healing period. These tissues lose strength because their normal blood supply is disrupted by the transplantation procedure. During the healing process these transferred tissues eventually become revascularized and capable of regaining their strength. However, until the strength of the tissues is recovered, they must be protected from carrying normal loads. Therefore, these procedures are accompanied by long rehabilitation regimens.
The concept of tissue augmentation has been developed in order to protect transferred tissues used in ligament reconstruction by a mechanism of load sharing. Such load sharing is achieved by suturing an implant in parallel to the transferred tissue. An example of an augmentation device currently in clinical use is the 3M KENNEDY LAD Ligament Augmentation Device.TM. (LAD), described by McPherson et al. in "Experimental Mechanical and Histological Evaluation of the Kennedy Ligament Augmentation Device, Clinical Orthopaedics and Related Research, June, 1985, Volume 196, pp. 186-195.
A critical aspect of tissue augmentation is the recognition that long-term remodeling of the transferred tissue depends on the tissue carrying a portion of the load. Biological tissues remodel according to the loads which they carry. Therefore, in order to develop a strong biological reconstruction, the transferred tissue must carry some of the load. This load sharing is achieved with the LAD by anchoring the LAD to bone at only one end. The other end is sutured to the transferred tissue. Thus, the load in the ligament reconstruction is carried from one bone, through the LAD, then into the transferred tissue, and finally into the other bone. If the LAD were anchored directly to bone at both ends of the device, the load would be carried from one bone to the other bone primarily through the LAD, with the transferred tissue shielded from carrying any significant load. Such stress shielding would result because the LAD consists of a material which has a greater axial stiffness than the tissue, and when two parallel members are of greatly different stiffnesses, the stiffer member will carry most of the load in proportion to the difference in stiffnesses. Excessive stress shielding may prevent optimal long term tissue reorganization.
A potential drawback with the current ligament augmentation method using the LAD relates to the fact that the entire length of the tissue is not augmented, thus leaving a possible weakness in the unaugmented region. Another drawback with the LAD is that free grafts, which are especially weak at their attachment sites, are only augmented at one of the attachment sites.
Some clinicians have attempted to overcome the aforementioned drawbacks by attaching the LAD to bone at both ends, thereby reinforcing the transferred tissue along its entire length. Then later, after some healing of the tissue has occurred, the attachment at one end is removed in a second surgical procedure in order to prevent long term stress shielding of the tissue. While some success has been achieved with this method, it involves the undesirable necessity of exposing the patient to the risks and trauma of a second surgical procedure.